This invention relates to a method for producing a negative image employing compositions containing a resin which is a photolabile, blocked imide polymer together with a sensitizing agent which is a photoacid generator capable of de-blocking the imide upon exposure to actinic radiation.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired. The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the sensitizer is not. Upon imagewise exposure of the coated substrate to actinic radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers.
It has been observed that existing negative acting photoresist films have a tendency to swell during the development because organic solvents are employed as developers. Consequently, it is difficult to form patterns having a narrow open gap between two pads of resist because the resist remaining behind swells such that the narrow gap is bridged over, and the pads will touch each other during development. Even though the resist film will shrink after being removed from the developer, in the locations where swollen resist contacted another part of the pattern a stringy or filmy bridge will be created between the two parts of the pattern.
Due to these swelling problems and because positive acting resists are known not to swell upon development, it has become accepted practice to utilize positive acting resists for high resolution microphoto-lithography.
While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence positive resists are the choice for the manufacture of densely packed integrated circuits. In many commercial applications, it is desirable to convert a high resolution quinone diazide type positive resist for a negative working application because for some patterns it would be more convenient to be able to use a negative resist. For example, a mask of a certain pattern may be more easily prepared, or might be more reliable to use if it were mostly dark with a few clear areas rather than if it were the opposite tone, namely, mostly clear with a few dark areas. That this might be the case can readily be imagined by considering the effect of dust particles on a mostly clear mask as opposed to a mostly dark mask. Any dust particle in the mask's clear area will be reproduced in the resist as a defect regardless of the tone of the resist. Therefore, the mostly clear mask has, proportionately, a greater area susceptible to particle contamination than does the mostly dark mask.
As a result, there is interest in the field of image reversal because of the utility of this process in practical device manufacturing. Among the practical aspects of image reversal are the elimination of the need for a dual set of complementary masks to do both positive and negative imaging, greater resolution and process latitude than with positive imaging alone, reduction in standing wave effects, and higher thermal stability. In this regard, several methods are known in the art for such image reversal. For example, it is known that conventional positive acting photoresists comprised of novolac binder resin and diazoketone or diazoquinone photosensitizer can be caused to yield negative images as disclosed in U.S. Pat. No. 4,104,070.
Conventional image reversal novolak photoresists are limited in that they cannot be exposed at short ultraviolet wavelengths, i.e. less than about 300 nm, to give high resolution images because of the intense absorbency of novolak resin in this region. This invention circumvents this problem by using different binder resins. The sensitizers are compounds which produce a strong acid upon photolysis, while the resins are polymers selected for their UV-transparency and other physical properties. The resins have as an integral part an acid labile group, the removal of which is capable of transforming the polymer from insoluble in aqueous developer to soluble in aqueous developer.